The present invention relates to a method for continuously forming a fiber mat for manufacturing fiberboard according to the dry method.
When producing fiberboard according to the dry method (MDF, HDF, LDF, etc.) a fiber mat is formed on a moving transport means and subsequently pressed in a hot pressing operation. The mat can be formed as a single, homogenous, layer or in multiple layers, with different characteristics for fiber, resin, moisture, etc., and such a fiber mat can also be formed on top of other types of mats, such as OSB, wafer board, particle board, etc., to create a smooth surface layer.
Forming of the mat is usually performed in a passive way, i.e. by merely spreading the fibers onto a moving belt or wire and then adjusting the thickness of the mat by removing fibers from the upper surface of the mat. For instance, the forming can be performed by carrying the fibers in an airstream from a fiber bin and depositing them onto a wire screen while sucking the air through the mat and screen, or by purely mechanical deposition of the fibers, after being metered from a dosing bin, onto a number of spreader rolls, which spread the fibers down onto a forming belt, forming a mat on the belt. There are also systems available which include combinations of pneumatic and mechanical systems.
The mat so formed is in each of these cases adjusted by means of a scalper roll system to obtain an even upper surface and more important a correct mat weight. This scalper operation results in a certain amount of the fibers already formed being pneumatically transported back to the fiber bin. The amount of fibers so recirculated amounts to about 20 to 40% of the final fiber mat for a pneumatic system, and 5 to 15% for a mechanical one. This operation thus increases the load onto the former itself, requires a pneumatic system and electric energy for the transport and will finally degrade the fibers, due to mechanical treatment when passing the transport fan, aging of the resin due to the longer dwell time until hot pressing and uncontrolled change of the moisture content of the material when recirculated.
The distribution of fibers crosswise when using the pneumatic type of former is carried out by means of a mechanical nozzle or impulse air pushing the falling fibers to one side or the other. This method is very sensitive to changes in the pneumatic balances, and requires frequent observation and adjustment.
When using the mechanical type of former, most of the crosswise distribution is determined by the spreading of fibers into the dosing bin, and little can be done when the mat is formed.
A conventional mechanical former is disclosed in U.S. Pat. No. 5,496,570. This patent shows a bed of forming rolls working above the mat for spreading the fibers. This patent also shows a typical scalper roll used for adjusting the mat surface.
International Application No. WO 96/16776 shows a device for levelling a particulate material web or mat. This device consists of elliptical disks which are used to even out the upper surface of a formed mat with the aim of diminishing the scalping.
Co-pending Swedish Patent Application No. 9800209-0 discloses a method and an apparatus for improving the forming operation and eliminating the scalping off operation by means of a set of rotating forming rolls with forming blades working in the deposited material. It has been found to be essential to control the position of the last forming roll as described in this co-pending application.
It is an object of the present invention to provide a method for controlling the last roll in the set of rolls as described in the above mentioned Swedish patent application.
In accordance with the present invention, this and other objects have now been realized by the invention of a method for continuously forming a fiber mat having an upper surface comprising metering fibrous material, depositing the metered fibrous material onto a moving surface, the moving surface moving in a predetermined direction, forming the fiber mat from the fibrous material by applying a plurality of adjustable rotary rolls to the surface of the fiber mat, the plurality of adjustable rotary rolls being disposed along the predetermined direction and including a final adjustable rotary roll in the predetermined direction, whereby a bulb of the fibrous material is formed prior to the final adjustable rotary roll in the predetermined direction and the final adjustable rotary roll defines the upper surface of the fiber mat, detecting the size of the bulb of the fibrous material, and controlling the vertical position of the final adjustable rotary roll based on the size of the bulb of the fibrous material. In a preferred embodiment, the detecting of the size of the bulb of the fibrous material comprises measuring the size of the bulb by mechanically contacting the surface of the bulb.
In accordance with one embodiment of the method of the present invention, detecting of the size of the bulb of the fibrous material comprises measuring the size of the bulb without contacting the surface of the bulb.
In accordance with another embodiment of the method of the present invention, the method includes rotating the final adjustable rotary roll by means of a motor, and wherein detecting of the size of the bulb of the fibrous material comprises using a signal from the motor. Preferably, the signal from the motor comprises the motor current, and the method includes treating the motor current in a load transmitter.
In accordance with another embodiment of the method of the present invention, detecting of the size of the bulb of the fibrous material comprises detecting the difference in height between the height of the final adjustable rotary roll and the height of the surface of the fiber mat formed thereby. In a preferred embodiment, detecting of the difference in height between the height of the final adjustable rotary roll and the height of the surface of the fiber mat comprises measuring the height of the surface of the fiber mat by mechanically contacting the surface of the fiber mat.
In accordance with another embodiment of the method of the present invention, detecting of the difference in the height of the final adjustable rotary roll and the height of the surface of the fiber mat comprises measuring the height of the surface of the fiber mat without contacting the surface of the fiber mat.
The objects of the present invention are carried out by means of a bulb of material which is formed in front of the last roll. Control of the vertical position of the last roll can be obtained by controlling the size of the bulb, directly by indicating the height or volume of the bulb of fiber material or, alternatively, indirectly by indicating the difference in height between the last roll and the mat after the last roll. The indication in the form of a signal can be used for closed loop control of the vertical movement of the last roll.
Thus, it has been found that the volume of the fiber bulb in front of the last roll represents a buffer volume of fiber material. If fiber material is lacking in the incoming mat to the last roll, i.e. if there is a xe2x80x9cholexe2x80x9d or indentation in the mat, then this buffer can be used to fill this xe2x80x9cholexe2x80x9d, as long as the buffer volume is sufficient. In addition, the fact that the roll is throwing material in both directions will help to supply material to sections of the mat with a lack of material.
If a section of the mat with too high a local mat thickness approaches the last roll, this excessive material is absorbed by the bulb, and is also thrown sideways so that it is spread over the width of the mat. This is valid until the bulb reaches a height such that excessive fibers are thrown over the roll on top of the outgoing mat.
Thus, it has shown to be essential for good forming according to the present invention to maintain good control of the height of this fiber bulb. Too low a bulb height, i.e. too high a roll setting, will result in xe2x80x9cholesxe2x80x9d in the formed mat, while too high a bulb height, i.e. too low a roll setting, will result in fibers being thrown over the last roll, thereby creating an uneven mat.
Furthermore, it is also critical to adjust the position of the last roll continuously in relation to the mat density. This can also automatically be achieved by control of the bulb.
The alternative way of controlling the level of the last forming roll is to use the difference in height between the last roll and the mat height after the last roll. This difference is also related to the size of the bulb before the last roll. If the last roll is set too high, then the difference is large and, at the same time, the fibers pass under the last roll without a bulb being created. If the last roll is set too low, then the difference is small or even negative and a large bulb is created, resulting in fibers being thrown over the last roll, thereby adding to the mat height. Thus, also by means of this indirect indication of the size of the bulb, it is possible to maintain a suitable bulb size in order to control the vertical position of the last forming roll, and thereby obtain the equalizing effect on the mat forming as described above.